public class SynchronizedCache
{
  private ReaderWriterLockSlim cacheLock = new ReaderWriterLockSlim();
  private Dictionary<int, string> innerCache = new Dictionary<int, string>();

  public string Read(int key)
  {
    cacheLock.EnterReadLock();
    try
    {
      return innerCache[key];
    }
    finally
    {
      cacheLock.ExitReadLock();
    }
  }

  public void Add(int key, string value)
  {
    cacheLock.EnterWriteLock();
    try
    {
      innerCache.Add(key, value);
    }
    finally
    {
      cacheLock.ExitWriteLock();
    }
  }
...

private Dictionary<int, string> innerCache = new Dictionary<int, string>();

public string Read(int key)
{
  readlock(innerCache) return innerCache[key];
}
public void Add(int key, string value)
{
  writelock (innerCache) innerCache.Add(key, value);
}